1. Field of the Invention
The present invention relates to a process for recovering catalyst components in production of aromatic carboxylic acids by liquid phase oxidation of aromatic hydrocarbons having substituents.
2. Description of the Prior Art
As the process for producing aromatic carboxylic acids, it is widely conducted that an aromatic hydrocarbon having substituents used as the raw material is oxidized in the liquid phase with a gas containing molecular oxygen at a high temperature and a high pressure in acetic acid as the solvent in the presence of a catalyst such as cobalt and manganese and a bromine compound.
A mother liquor obtained after separating crystals of the aromatic carboxylic acid from the reaction fluid of the liquid phase oxidation by filtration or centrifugal separation (this mother liquor will be referred to as the mother liquor of liquid phase oxidation, hereinafter) contains useful components of the catalyst such as cobalt, manganese and bromine. It is desired that these components are efficiently reused.
However, the mother liquor of liquid phase oxidation contains not only acetic acid used as the solvent and the components of the catalyst but also water as the byproduct of the reaction and intermediate products of the reaction. Moreover, the apparatus used for the reaction is gradually corroded due to corrosive components such as bromine and metal components such as iron, chromium and nickel are eluted into the mother liquor of liquid phase oxidation (the metal components eluted into the mother liquor will be referred to as the eluted metals, hereinafter).
Many of the intermediate products of the reaction and the eluted metals adversely affect the oxidation reaction and cause deterioration in the quality of the product and a decrease in the yield. Therefore, direct recycling and reuse of the mother liquor of liquid phase oxidation is limited.
The components of the catalyst can be recovered from the mother liquor of liquid phase oxidation in accordance with various processes. In one of such processes, for example, a portion of the mother liquor of liquid phase oxidation is concentrated in a concentration tank, organic components in the concentrated product are incinerated and the components of the catalyst are recovered from the residual ashes. In another process, the components of the catalyst are recovered by extracting from the residue in the concentration tank with water as the solvent. In still another process, oxalic acid is added to the mother liquor of liquid phase oxidation so that the metal components are fixed as the salts of oxalic acid and the components of the catalyst are recovered by decomposition of the salts of oxalic acid with the air. The above processes have drawbacks in that a great amount of labor and cost are required.
As the process for recovering components of the catalyst from a mother liquor of oxidation, processes for recovering the components of the catalyst via adsorption to an ion exchange resin are proposed (Japanese Patent Application Laid-Open Nos. Showa 53(1978)-104590 and Heisei 10(1998)-15390).
In the above process for recovering the components of the catalyst via adsorption to an ion exchange such as the process disclosed in Japanese Patent Application Laid-Open No. Heisei 10(1998)-15390, fine crystals present in the mother liquor of oxidation are removed by filtration using a filter or the like. Then, the components of the catalyst such as cobalt and manganese are converted into bromine complexes using bromine ion present in the clear mother liquor obtained after the filtration and the components of the catalyst are adsorbed to an ion exchange resin in the form of the bromine complexes. The components of the catalyst are recovered by elution with an aqueous solution of acetic acid or water. This process has a drawback in that the liquid obtained by the elution contains eluted metals besides the components of the catalyst and the eluted metals adversely affect the oxidation.
The present invention has an object of providing a process for efficiently recovering components of the catalyst while the amount of eluted metals contained in the recovered components of the catalyst is suppressed in the production of aromatic carboxylic acids by liquid phase oxidation of aromatic hydrocarbons having substituents in the presence of a catalyst comprising cobalt, manganese and bromine ion in a solvent containing acetic acid.
As the result of extensive studies by the present inventors to overcome the above drawbacks, it was found that the amount of the eluted metals can be suppressed by adding a small amount of an ion of an alkali metal to a mother liquor of liquid phase oxidation when cobalt, manganese and bromine are recovered from the mother liquor of liquid phase oxidation. The present invention has been completed based on the knowledge.
The present invention provides a process for recovering components of a catalyst from a mother liquor of liquid phase oxidation in production of aromatic carboxylic acids by liquid phase oxidation of aromatic hydrocarbons having substituents with a gas containing oxygen in a presence of a catalyst comprising cobalt, manganese and bromine in acetic acid or acetic acid containing water as a solvent, which process comprises adding a compound generating an ion of an alkali metal to the mother liquor of liquid phase oxidation obtained after separating crystals of the aromatic carboxylic acid from a reaction fluid of the liquid phase oxidation, bringing the mother liquor containing the compound into contact with an ion exchange resin so that the components of the catalyst are adsorbed to the ion exchange resin and recovering the components of the catalyst comprising cobalt, manganese and bromine ion by passing an elution liquid through the ion exchange resin.